This invention relates to an improved process for regenerating fluidizable catalytic cracking catalyst. In particular, it is related to a method of operating the regenerator of a fluid catalytic cracking unit (FCCU) having a single fluidized dense catalyst phase wherein coke-contaminated fluidizable catalytic cracking catalyst is contacted with an oxygen-containing regeneration gas in order to obtain a regenerated catalyst having a low carbon content while producing a regenerator effluent flue gas having a carbon monoxide content substantially lower than obtained heretofore.
The fluidized catalytic cracking of hydrocarbons is well-known in the prior art and may be accomplished using a variety of continuous cyclic processes which employ fluidized solids techniques. In such fluid catalytic cracking processes hydrocarbons are converted under conditions such that substantial portions of a hydrocarbon feed are transformed into desirable products such as gasoline, liquified petroleum gas, alkylation feedstocks and middle distillate blending stocks with concomitant by-product formation of an undesirable nature, such as gas and coke. When substantial amounts of coke deposition occur, reduction in catalyst activity and, particularly, catalyst selectivity results thereby deterring hydrocarbon conversion, reducing gasoline production and simultaneously increasing the production of less desired products. To overcome such catalyst deactivation through coke deposition, the catalyst is normally withdrawn from the reaction zone and passed to a stripping zone wherein entrained and absorbed hydrocarbons are initially displaced from the catalyst by means of stripping medium such as steam. The steam and hydrocarbons are removed and the stripped catalyst is passed to a regeneration zone where it is contacted with an oxygen-containing gas to effect combustion of at least a portion of the coke and thereby regenerate the catalyst. Thereafter, the regenerated catalyst is reintroduced to the reaction zone and therein contacted with additional hydrocarbons.
Generally, regeneration processes provide a regeneration zone wherein the coke-contaminated catalyst is contacted with sufficient oxygen-containing regeneration gas at an elevated temperature to effect combustion of the coke deposits from the catalyst. Most common of the regeneration processes are those wherein the contacting is effected in a fluidized dense catalyst phase in a lower portion of the regeneration zone constituted by passing the oxygen-containing regeneration gas upwardly through the regeneration zone. The space above the fluidized dense catalyst phase contains partially spent regeneration gases and catalyst entrained by the upward flowing regeneration gas. This portion of the regeneration zone is generally referred to as the dilute catalyst phase. The catalyst entrained in the dilute catalyst phase is recovered by gas solid separation cyclones located in the upper portions of the regeneration zone and is returned to the fluidized dense catalyst phase. Flue gas comprising carbon monoxide, other by-product gases obtained from the combustion of the coke deposits, inert gases such as nitrogen and any unconverted oxygen is recovered from the upper portion of the regeneration zone and a catalyst of reduced carbon content is recovered from a lower portion of the regeneration zone.
In the regeneration of catalytic cracking catalyst, particularly high activity molecular sieve type cracking catalysts, it is desirable to burn a substantial amount of coke from the catalyst such that the residual carbon content of the regenerated catalyst is very low. A carbon-on-regenerated-catalyst content of about 0.15 weight percent or less is desirable. Cracking catalysts with such a reduced carbon content enable higher conversion levels within the reaction zone of the FCC unit and improved selectivity to gasoline and other desirable hydrocarbon products.
Most of the prior art processes for regenerating fluid catalytic cracking catalyst generally involve contacting the coke-contaminated catalyst in the fluidized dense catalyst phase at a temperature of from about 1100.degree. F. to about 1200.degree. F. for a sufficient period of time to reduce the carbon content of the catalyst to the desired level. Such processes are undesirable in that the carbon content of the regenerated catalyst is generally reduced only to a level of from about 0.3 to about 0.5 weight percent and because a flue gas is obtained containing large amounts of carbon monoxide which must be treated prior to discharge into the atmosphere.
It is known that increasing the temperature of the fluidized dense catalyst phase will reduce the residual carbon level of the regenerated catalyst. However, processes in which the temperature of the fluidized dense catalyst phase exceeds about 1200.degree. F. generally involve elaborate modifications to counteract the effects of after-burning in the dilute catalyst phase. By after-burning is meant the further oxidation of carbon monoxide to carbon dioxide in the dilute catalyst phase. Whenever after-burning occurs in the dilute catalyst phase, it is generally accompanied by a substantial increase in the temperature due to the large quantities of heat liberated. In such circumstances the dilute phase temperature may exceed about 1500.degree. F. and, in severe cases, may increase to about 1800.degree. F. or higher. Such high temperature in the dilute catalyst phase are deleterious to the entrained catalyst present in the dilute catalyst phase and result in a permanent loss of catalytic activity, thus necessitating an inordinately high rate of catalyst addition or replacement to the process in order to maintain a desired level of catalytic activity in the hydrocarbon reaction zone. Such high temperatures are additionally undesirable because of the damage which may result to the mechanical components of the regeneration zone, particularly to cyclone separators employed to separate the entrained catalyst from the flue gas.
It is known that commonly employed catalytic cracking catalysts such as amorphous silica-alumina, silica-alumina zeolitic molecular sieves, silica-alumina zeolitic molecular sieves ion-exchanged with divalent metal ions, rare earth metal ions, etc., and mixtures thereof, are adversely affected by exposure to excessively high temperatures. At temperatures of approximately 1500.degree. F. and higher, the structure of such catalytic cracking catalyst undergo physical change, usually observable as a reduction in the surface area with resulting substantial decrease in catalytic activity. Consequently, it is desirable to maintain the temperatures within the regeneration zone at levels below which there is any substantial physical damage to the catalyst.
Known methods for regenerating fluid catalytic cracking catalysts to low carbon contents, while avoiding excessively high dilute catalyst phase temperatures, are generally unsatisfactory. In some processes a cooling medium which may comprise steam, liquid water, unregenerated catalyst, hydrocarbon oil, flue gas, etc., is brought into heat exchange contact with the dilute catalyst phase either to absorb the heat liberated by after-burning which may occur therein or to prevent the occurrence of after-burning. See, for example, U.S. Pat. Nos. 2,382,382; 2,580,827; 2,454,373; 2,454,466; 2,374,660; 2,393,839; and 3,661,799.
Other methods employ multiple catalyst regeneration zones to provide sufficient residence time for contacting the coke-contaminated catalyst with an oxygen-containing regeneration gas to burn the coke deposits therefrom at a temperature at which after-burning will not occur. See, for example, U.S. Pat. Nos. 3,563,911; 2,477,345; 2,788,311; 3,494,858; 2,414,002; and 3,647,714. Still other methods such as those disclosed in U.S. Pat. Nos. 2,831,800 and 3,494,858 teach multiple zone regeneration of coke-contaminated catalyst, but are silent with respect to control of dilute phase temperatures. Still another approach employed involves indirect heat exchange such as steam generation coils employed in the fluidized dense catalyst phase.
All of the above methods are unsatisfactory in that the processes involved cumbersome additional processing steps for absorbing heat liberated due to after-burning in the dilute catalyst phase or require expensive facilities for the treatment of the regeneration flue gas stream, because of the avoidance of after-burning in the regeneration zone and the resultant substantial carbon monoxide content in the flue gas, generally ranging from about 2 to about 6 volume percent, or higher.